In WebFig the `Prefix` text box is not editable, and always defaults to `::/64`.
It's just taken me a hours to think about running WinBox in Wine in order to see whether it is different.
Spoiler: it is.
As you can see, in WebFig the `Prefix` is grey and not editable.
But in WinBox the `Prefix` is editable.
Please can you fix the bug in WebFig.